Variables psicofisiológicas en el dolor crónico

The media used in constructed wetlands is named substrates or aggregates. Wetlands media could be sand, gravel, rocks or organic material such as soil and compost which provide the primary support for the wetlands plants and microorganism growth, enhancing biodegradation of wastewater pollutants, in addition to its impact on system hydrology mechanisms (Tietz et al., 2007; Meng et al., 2014). Moreover, substrates can remove contaminants from the wastewater by ion exchange / non-specific adsorption, specific adsorption / precipitation and complexation (Dordio & Carvalho, 2013; Ge et al., 2015). However, the chemical composition of wetland media can affect the system efficiency. For example, soil of low nutrient content will lead the plants in the system to uptake the nutrients from the applied wastewater directly. Also, the gravel substrate in the system should be washed from time to time to enhance the filtration rate and reduce the clogging of system media. Furthermore, using a gravel substrate with a reed system will improve the nitrification process rates, while the use of soil media with a reed system in the wetlands will increase the denitrification rate as discussed by Markantonatos et al. (1996). Moreover, substrate size has an important role in the system mechanism as it may affect the surface area for growing the biofilm in addition to the system pores blockage probability.

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For instance, Meng et al (2014) reported that an excessively large aggregate size will reduce the surface area for microorganisms to grow, while Brix and Arias (2005) indicated that the small-sized-grain media will support the growth of biofilm by increasing the available surface area with a high possibility of clogging the system pores. Furthermore, Hoffman et al. (2011) and Meng et al. (2014) concluded that the hydraulic loading rate in wetland systems particularly subsurface flow types can be affected directly by wetland aggregate porosity, as the clogging of wetlands media is a common problem in such systems affecting the system performance, especially when using unsuitable media pores for the applied organic load.

However, the media which is used in the wetland is dependent on the purpose for which the wetlands are designed. This media can be varied from fine grain to field stones. Using coarse grained media in the wetland systems will increase the hydraulic conductivity and reduce the probability of system clogging while the fine media will remove the suspended solids and turbidity well with a high potential for clogging to occur in the system (Sundaravadivel & Vigneswaran, 2009).

Under saturation conditions, the pores in the wetland substrate will be filling with water instead of air. In this case the dissolved oxygen available in the water will be consumed by microbes. Since this oxygen will be more than that restored during the circulation phase, the media will become anoxic. Moreover, the substrate will be anaerobic under inundation conditions (Scholz, 2006, 2010; Stefanakis et al., 2014). Using a sand and gravel mixture in the wetland substrate is recommended as this can improve the system behaviour in terms of hydraulic conductivity and contaminants removal (Stottmeister et al., 2003).

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Moreover, using fine grained instead of large-grained media in a wetland system is preferable as it will provide better conditions for microorganism growth and subsequently improve pollutant biodegradation (Dordio & Carvalho, 2013) but at the same time the fine aggregate will lead to clogging the media (Brix & Arias, 2005; Song et al., 2015). Langergraber et al. (2003) reported that using compound layers of gravels arranged by size increment from the top is suggested, however, their results showed that the clogging in such a system is highly likely.

Other studies showed that using an anti-sized reed bed system instead of the traditional mono-sized is very effective in terms of pollutant removal from heavily contaminated wastewater (Sun, Zhao, & Allen, 2007), while Song et al. (2015) indicated that the using of large-size packing media will result in high removal of chemical oxygen demand, ammonia and nitrogen while reducing the chance of system clogging as evaluated in their vertical flow wetland systems. Moreover, several studies have been undertaken to assess the impact of different substrate media used to improve contaminant adsorption capacity.

For example, Menge et al. (2014) confirmed the results obtained from previous studies (Saeed & Sun, 2011; Tee et al., 2012; Saeed & Sun, 2013) which assessed using different media substrates, such as organic mulch and rice husk, on system efficiency. The results showed that these substrates enhanced the nitrogen removal due to organic carbon content. However, these results contradicted others regarding the use of expensive media to improve the wetland system performance. For instance, using granular activated carbon did not increase the adsorption capacity of constructed wetland media as shown by Scholz and Xu (2002).

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Moreover, using zeolite and bauxite substrates did not show a substantial enhancement in wetland system efficiency as reported by Stefanakis and Tsihrintzis (2012). Table 2.1 displays the most common substrates used in constructed wetland systems.

Table 2.1: Common substrate types used in constructed wetland systems with source.

Substrate type

Natural material Industrial by products Artificial products Sand (Saeed & Sun,

2013)

Slag (Cui et al., 2010) Activated carbon (Ren et al., 2007)

Gravel (Calheiros et al., 2008)

Fly ash (Xu et al., 2006) Light weight aggregates (Saeed & Sun, 2012) Clay (Calheiros et al.,

2008)

Coal cinder (Ren et al., 2007) Compost (Saeed & Sun, 2012) Calcite (Ann et al., 1999) Alum sludge (Babatunde et al.,

2010)

Calcium silicate hydrate (Li et al., 2011)

Marble (Arias et al., 2001)

Hollow brick crumbs (Ren et al., 2007)

Ceramsite (Li et al., 2011) Vermiculite (Arias et al.,

2001)

Moleanos limestone (Mateus et al., 2012)

Bentonite (Xu et al., 2006)

Wollastonite tailings (Hill et al., 1997)

Dolomite (Ann et al., 1999)

Oil palm shell (Chong et al., 2013)

Limestone (Tao & Wang, 2009)

Shell (Seo et al., 2005) Shale (Saeed & Sun, 2012)

Peat (Saeed & Sun, 2012) Wollastonite (Brooks et al., 2000)

Maerl (Saeed & Sun, 2012)

Zeolite (Bruch et al., 2011)

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